UBC Theses and Dissertations
Investigating the role of sucrose synthase in Arabidopsis thaliana and hybrid poplar Chen, Tsung Jeng
Sucrose synthase (SuSy) cleaves sucrose to produce fructose and UDP-glucose, the latter serving as the precursor for cellulose biosynthesis. Sucrose synthase often exists as many isoforms in plants; for example, Arabidopsis has six isoforms. Along with it role in routine metabolism, sucrose synthase is also important during stress conditions. For example, during hypoxia plants can accumulate reduced substances, often to phytotoxic levels, as well as displaying energy deprivation stemming from the reduced levels of oxygen. Given that SuSy is more energetically efficient than invertase it therefore plays important roles in combatting hypoxic stress. This thesis describes transgenic studies involving functional complementation of sus1/sus4 Arabidopsis mutants, as well as overexpression of exogenous SuSy in wild-type Arabidopsis plants under hypoxia to determine if the Populus trichocarpa (Pt) SuSy1 and 2 genes are functionally conserved orthologs of the Arabidopsis SuSy1 and SuSy4 isoforms. Real-time PCR and microscopy were employed quantify and observe the YFP reporter, respectively. Changes in Arabidopsis plant stem height under hypoxia documented abnormalities in plant growth. Under hypoxia, sus1/sus4 Arabidopsis plants displayed slowed stem growth, while all transgenic lines (complemented sus1/sus4 and over-expressed wild-type plants) grew normally. Histochemical staining of stem cross sections, demonstrated that the sus1/sus4 mutants subject to hypoxia displayed thickened xylem cell walls, and possessed increased in cell wall glucose content and hemicellulose-derived carbohydrates. Together, the findings suggest the conserved functionality of the PtSuSy1 and PtSuSy2 orthologs. Real-time PCR was employed to determine the transcript abundance of PtSuSy1 and PtSuSy2 in poplar in several tissues (phloem, cambium, and leaf) collected over 24 hours and a complete growing season. Examination of SuSy in the three tissues revealed a minor role for SuSy1 and SuSy2 in source tissues as levels of SuSy expression were consistently lower in both diurnal and annual samples. In all samples, PtSuSy1 was consistently more highly expressed than PtSuSy2 suggesting a more essential role in sucrolysis during active growth. The role of SuSy in sucrolysis was further reinforced as the expression of SuSy in both diurnal and annual tissues was constantly associated with plant growth during which UDP-glucose would be in high demand.
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